The continuous development of magnetic recording disk drives results in ever increasing data storage densities in the storing layers. To read and write the magnetic signals, the read and write heads have to be kept in ever-closer distance to the rotating disc surface where the storing layers are deposited.
The read and write heads are typically integrated in the so-called sliders, which provide specifically designed three-dimensional features on their bottom side that is next to the disk surface. These three-dimensional features utilize the viscosity and kinetic energy of a rotating air stream induced by the spinning disk to lift the sliders on a predetermined fly height during the hard disk operation.
The viscosity of the air stream depends mainly on the air temperature and the air pressure. The kinetic energy of the rotating air stream depends on its velocity relative to the slider and subsequently on the rotational speed of the hard disk.
The bottom side performs the function of an air bearing in closest proximity to the disk surface. As a result fly heights in the nanometer range can be implemented.
Such small fly heights require high precision of the disk surface since even the smallest surface inconsistencies result in a contacting of the slider with the fast moving disk surf ace. Even though the utilized fabrication processes provide for sufficient surface evenness of the hard disk, special wear-in procedures are commonly performed to eliminate eventual and/or recognized surface unevenness. These wear-in procedures are typically performed by reducing the fly height below the operational level and moving the slider over the surface until no contacting is recognized anymore. The slider, which is made of a relatively hard material is thereby utilized as an abrasive tool to remove any interfering surface inconsistencies from the relatively soft top layers of the hard disk.
The fly height is typically reduced by changing the rotational speed of the hard disk and/or by changing the air pressure.
A number of U.S. Patents discloses variations of the hard disk wear-in procedure, which is commonly referred to as burnishing.
U.S. Pat. No. 5,696,643 and U.S. Pat. No. 5,863,237, for instance, describe methods to burnish away topographic irregularities from the disk surface. After recognizing an surface irregularity via a thermal contacting signal, the rotational speed of the hard disk is reduced and the fly height of the read/write head is lowered. The burnishing is performed over a certain time period, during which the height of the surface irregularities is continuously reduced. After finishing the disk burnishing the interference signals no longer occur during operational rotation of the hard disk.
Japanese Patent JP 06309636 describes a similar burnishing method, except that the read/write head is lowered by reducing the air pressure under which the hard disk drive operates.
The thermal contacting signal results from a dynamic resistance change in the read head, which is thermally induced by the frictional energy created during the contacting of the head.
The dynamic resistance change itself may be recognized with various methods. In one method, it is recognized during the regular read operation of the hard disk. This requires a fully functioning hard disk drive, including a partitioned hard disk. U.S. Pat. No. 5,751,510 describes such a method.
In another method, the dynamic resistance change is obtained by the read/write head without reading any data from the hard disk. In such a case, an electrical stimulus voltage is applied to the read head. This method can be performed at an earlier hard disk fabrication stage since it does not require operational data read from the hard disk surface. A calibration signal and/or a calibration value has to be obtained for a known non-interference condition. U.S. Pat. No. 5,806,978, for instance, describes such a method.
With continuously decreasing fly heights a contacting and non-contacting operational conditions in the head/disk interface become less and distinctly able. Read/write heads operate typically with their air bearing surface in, an angulated orientation relative to the disk surface. The microscopic air bearing features are typically fabricated with a common protrusion direction normal to the substrate plane, which results in essentially coplanar surfaces and linear edges. The design of the air bearing surface defines the primary contacting edge, which initially contacts the moving disk surface. In the case where the front portion of the air bearing surface is raised sufficiently, the primary contacting edge becomes the front edge with the read and write elements. In such a case, the contacting of the slider during the regular hard disk operation occurs mainly with the slider front edge.
The linear contacting of the slider with the primary contacting edge results in relatively high surface pressures, which result in wear of the disk surface and/or the slider. As a result of disk wear, debris may adhesively build up on the primary contacting edge. Since it is desirable to have the read/write heads in closest proximity to the disk surface, they are preferably in an area adjacent to the primary contacting edge. Debris built-up alters the read and write characteristic of the heads and needs to be prevented. U.S. Pat. No. 6,088,199, for instance, discloses an abrasive section placed on the hard disk to remove eventual debris built-up on the slider. The patent does not prevent debris from building up, however. It provides only a cleaning method.
Wear in the head/disk interface related to operational slider contact is explored in a number of scientific disclosures.
In IEEE Trans. Magn. (USA) vol 34, no. 4, pt. 1, p. 1714–16, a conference/journal paper is disclosed, which describes the abrasive wear and adhesion of the slider surface.
In the 1996 AME/STLE Tribology Conference (TRIB-Vol. 6) p. 17–23, a conference paper is disclosed, which describes new techniques for evaluating slider wear and burnishing of the head/disk interface.
Further, in the Proceedings of the SPIE—The International Society of Optical Engineering (USA) vol. 2604 p. 236–43, contact force measurements at the head/disk interface for contact recording heads in magnetic recording are disclosed and correlated to the burnishing in the head/disk interface.
Finally, in the Journal of Materials Research vol. 8, no. 7 p. 1611–28, friction and wear studies of silicon in sliding contact with thin-film magnetic rigid disks are disclosed.
The ever decreasing fly heights make the limitations described in the above scientific paper increasingly stringent.
The present invention addresses these limitations and provides a solution for them.